Electric motor

ABSTRACT

A miniature PMDC motor  10  has a permanent magnet stator comprising two arcuate ceramic magnets  20  fitted to a can-like housing  11  and circumferentially separated by interdisposed rubber magnets  21  wedging or nipping the ceramic magnets  20  in place within the housing  11  without the use of circumferential magnet stops formed in the side of the housing.

This invention relates to electric motors and in particular, tominiature electric motors having a permanent magnet stator.

PMDC motors are very common and have many uses and applications. Someapplications require motors with a relatively high power with smallphysical size while some applications require the motor to have lowcogging.

Cogging can be reduced by shaping the rotor poles and/or the statorpoles. As the stator is a permanent magnet, the shaping of the statorpoles means shaping the magnets or the magnetic field which can beshaped during the charging of the magnets. This is usually only possiblewith ring magnets but ceramic ring magnets are very expensive and rubberring magnets are not as strong.

When using ceramic magnets, it is usual to use two arcuate ceramicmagnets which are circumferentially located with respect to the can-likehousing by stops which engage one longitudinal side of each magnet andone or more springs located between and bearing against the otherlongitudinal edge of the magnets, pressing the magnets into contact withthe stops.

A similar arrangement is used with four arcuate magnets and two steps ofstops.

Once installed in the housing, the magnets are charged. However, the gapbetween the ceramic magnets represents a gap in the magnetic field ofthe stator and any shaping of the magnetic field must take this gap intoconsideration.

The rubber ring magnet, on the other hand, requires no springs and nostops pressed into the wall of the housing. The magnet is held in placeby the magnet itself which expands when it is charged resulting in ahousing without side openings. However, there is a need for a housingwithout side openings which supports a more powerful magnet such as aceramic magnet.

Accordingly, the present invention provides a permanent magnet motorhaving a cylindrical housing containing a permanent magnet stator and awound rotor, wherein the permanent magnet stator comprises at least twoarcuate ceramic magnets circumferentially spaced by rubber magnets. Thepresent invention also provides a method of making such a motor.

BRIEF DESCRIPTION OF THE DRAWINGS

One preferred embodiment will now be described, by way of example only,with reference to the accompanying drawings, in which:

FIG. 1 illustrates a permanent magnet direct current electric motoraccording to the preferred embodiment of the present invention;

FIG. 2 is a longitudinal cross-section of the stator of the motor ofFIG. 1; and

FIG. 3 is a transverse cross-sectional view of the stator of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The motor shown in FIG. 1 is a miniature PMDC motor 10 having a two polepermanent magnet stator. The stator comprises a cylindrical can-likehousing 11 surrounding the permanent magnets and accommodating a woundrotor of which only a shaft 12 is visible. One end is closed by a moldedresin end cap 14 which supports motor terminals 15, brush gear and abearing. The other end of the housing 11 also supports a bearing for therotor shaft 12. Although shown as having one open end and one closedend, the housing could have two open ends, both of which are closed byseparate end caps.

The stator, more clearly shown in FIGS. 2 and 3, has two ceramic arcuatemagnets 20 placed inside the housing 11. There are no stops formed inthe housing to locate the magnets 20 circumferentially with respect tothe housing 11. A rubber magnet 21 is pressed into each gap betweenadjacent longitudinal edges of the ceramic magnets 20. The rubbermagnets 21 can be compressed to aid assembly but once charged, therubber magnets 21 swell, creating a firm interference fit between theceramic magnets 20 and the rubber magnets 21 fixing the magnets 20, 21to the housing 11.

The ceramic magnets 20 may be additionally supported by gluing themagnets to the housing 11. The housing 11 may have a notch 16 at one endfor aligning the magnets 20, 21 within the housing 11 during assemblyand subsequently, for aligning the end cap 14 with the housing 11 sothat the brushes are correctly aligned with the magnets to ensure thecorrect commutation angle.

Although a two pole stator has been shown in the example, the inventiveconcept can be applied to permanent magnet stators generally having twoor more poles and is particularly suited to four pole stators.

The motor thus constructed has a stator with a continuous magnetic fieldsimilar to that provided by a ring magnet but with higher energy ceramicmagnets at the pole centres and rubber magnets at the interface betweenthe poles allowing shape charging of the stator. This method also allowsfor the simple fixing of arcuate ceramic magnets to a motor housing andavoids the use of housing deforming magnet stop wings punched into theside of the housing.

The embodiment described above is given by way of example only andvarious modifications will be apparent to persons skilled in the artwithout departing from the scope of the invention as defined in theappended claims.

1-5. (Cancelled)
 6. A method of making a miniature electric motor,comprising the steps of: taking a deep drawn cylindrical housing andinserting two ceramic arcuate magnets and locating the ceramic magnetscircumferentially spaced against an inner circumferential wall of thehousing, inserting rubber magnets into the housing and placing therubber magnets against the inner circumferential wall of the housing tofill the circumferential gaps between the ceramic magnets, mounting awould rotor into the housing confronting the magnets, closing thehousing with an end cap, and applying a strong magnetic field across thehousing to charge the magnets.
 7. The method of claim 6 wherein the stepof charging the magnet is carried out before the rotor is fitted to thehousing.
 8. The method of claim 6 wherein an additional two ceramicarcuate magnets are circumferentially spaced against the innercircumferential wall of the housing and the rubber magnets are insertedbetween each pair of adjacent ceramic magnets to form a continuous ringof magnets.
 9. The method of claim 6 wherein the rubber magnets arecompressed before being inserted between the ceramic magnets.
 10. Themethod of claim 6 further comprising applying adhesive to a radiallyouter surface of the ceramic magnets before inserting the ceramicmagnets into the housing.